1. Field of the Invention
The invention relates to a method for testing of a measuring device arrangement which comprises at least one measuring device. The measuring device, based on the determination of at least one measured quantity, generates at least one output signal which can be tapped from a pick-off site as a pick-off signal. The invention furthermore relates to a measuring device arrangement with at least one measuring device and one pick-off site. Furthermore the invention relates to a test arrangement for testing of a measuring device arrangement.
2. Description of Related Art
In modern process automation, the safety of the control or regulation of processes and the reliability of the field devices which are used for this purpose (for example, sensors for determining and/or monitoring of measured quantities or process variables or actuators for control or regulation of processes) acquire high and increasing importance. In particular, for safety-critical applications, it is necessary for the measurement or display devices to produce the correct output signals, i.e., those which are appropriate to the monitored state and for these output signals to be correctly processed in subordinate or higher-level units.
In order to be able to obtain information about the reliability or safety of devices of process automation, function tests or tests of individual components or of entire systems are therefore carried out. One possibility is to start up different states of the system which is being monitored, and for example, also controlled by the field devices, and to evaluate the signals which have been generated by the measuring devices. Thus for example, tanks in which the levels of media are to be monitored are filled or emptied during a function test. On the one hand, this is to some extent very complex. On the other hand, it is not always possible or it must be prevented under certain circumstances that special states of the system to be monitored be present. For example, if it is critically necessary for the overfilling of a container with a dangerous substance to be recognized or prevented, this state cannot be started up in the manner of a test. Therefore, in tests the presence of states or values of the measured quantities which are associated therewith is preferably simulated by for example, corresponding signals being fed into the field devices.
International Patent Application Publication WO 2010/006897 A1 describes a method for testing of field devices and tackles the problem that the output signals produced during the function test of the measuring devices do not reflect the real measured values. Therefore, a test of the measuring devices is carried out in the time interval in which the measured quantities have fixed and known values. Preferably, this takes place at the time at which a test of the units which are subordinate to the measuring devices or of the entire system also takes place. Therefore, the time interval is used for the test in which a measurement is not necessary because it is known that the measured quantity does not change or because a genuine evaluation of the measurement signals dictated by the test of the higher-level units does not take place. The disadvantage here is that an inline measurement, therefore a measurement during process operation, is not possible since the actual measurement does not take place or is interrupted for the test.
In U.S. Pat. No. 5,481,200, the output of a field device which is connected to a two-wire current loop is monitored by the loop resistance being determined by switching of two different currents in the current loop. With this, faults of the field device or especially of the signal output will be recognized. The disadvantage here is that by switching different currents, the transmission of the current value belonging to the measured value cannot take place, i.e., the output of the actual measured value does not take place.
U.S. Patent Application Publication 2011/0148431 A1 describes how a current transformer, as part of a measuring device, for example, can be monitored. For this purpose, a test signal is sent to the input of the current transformer and the output signal is compared to a setpoint signal. The disadvantage here is that a test state which prevents functioning of the object which is to be tested is likewise induced.